1. Field of the Invention
The present invention relates to a manufacturing method of a silicon carbide semiconductor device and more particularly, to a manufacturing method of a silicon carbide Schottky barrier diode.
2. Description of the Background Art
It is important to select a Schottky metal and to stabilize forward characteristics in producing a silicon carbide Schottky barrier diode (hereinafter, referred to as SiC-SBD). In general, the Schottky metal includes Ti (titanium), Ni (nickel), Mo (molybdenum), and W (tungsten), but in a case where a Ti Schottky barrier diode is produced, when Ni is employed as an electrode on a back surface to form a Ni ohmic junction, there are following process characteristics and problem.
In other words, a SiC-SBD having a high withstand voltage at kV level has a configuration in which a Schottky electrode is formed on an n-type epitaxial layer formed on a silicon carbide substrate, but in this structure, an electric field is likely to concentrate at an edge part of a junction surface between the epitaxial layer and the Schottky electrode, so that a p-type termination structure is generally formed in an upper layer part of the epitaxial layer which corresponds to the edge part of the junction surface (Schottky junction surface) to alleviate the electric field concentration.
The p-type termination structure is a p-type impurity region formed around an electrode region of the SBD, and it is formed in such a manner that ions of a p-type impurity such as Al (aluminum) or B (boron) are implanted into an n-type epitaxial layer, and subjected to an activation annealing treatment by a high-temperature heat treatment at 1500° C. or higher.
In order to obtain the Schottky junction having preferable characteristics, it is necessary to remove an altered layer on a SiC surface generated during the high-temperature heat treatment, and as a technique to remove this altered layer, well-known techniques are disclosed in Japanese Patent Application Laid-Open Nos. 2008.53418, 2001.35838, 2004.363326, and 2007-141950.
As a method to remove the altered layer, Japanese Patent Application Laid-Open No. 2008-53418 discloses a method in which a SiC surface layer is sacrificially oxidized after an activation annealing treatment, a sacrificial oxide film having a thickness of 40 nm to less than 140 nm is formed in a surface layer, and the altered layer is removed together with the sacrificial oxide film.
Japanese Patent Application Laid-Open No. 2001-35838 discloses a method in which a natural oxide film is removed by a hydrofluoric acid treatment, and a SiC surface is cleaned by plasma etching with plasma of mixture gas of hydrogen and oxygen or plasma of gas containing fluorine atoms.
Japanese Patent Application Laid-Open No. 2004-363326 discloses that an altered layer having a thickness of about 100 nm is generated, and a method to remove the altered layer having the thickness of about 100 nm by etching in an atmosphere containing hydrogen, or a mechanical polishing.
Japanese Patent Application Laid-Open No. 2007-141950 discloses a method in which a thermal oxide film is formed on both front and back surfaces of a substrate in an oxygen atmosphere at 1160° C. after an activation annealing treatment, and just after that, the thermal oxide film is removed with a BHF solution.
In addition, regarding the production of the SiC-SBD, in order to stabilize the forward characteristics of the diode, especially a barrier height 4B or n value (ideal coefficient), conventionally well-known effective methods include a method in which a silicon carbide substrate is covered with a process protective film until just before a Schottky metal is formed, the process protective film is removed, and a Schottky metal (Ti in this case) is formed, and a method in which after the Schottky metal has been formed, it is subjected to a Ti sintering treatment at 400 to 600° C.
When the sintering treatment is not performed on the Ti Schottky barrier diode, the barrier height φB is largely varied in a range of 1.05 to 1.25 eV. In this case, characteristics of the individual SiC-SBD are largely varied, which may raise the problem that the characteristics cannot be aligned when a switching element or a power module or the like is constituted together with a silicon IGBT or a SiC-MOS FET, and the problem that a rate of non-defective products is lowered when a non-defective product selection standard is tightened. Therefore, it is important to produce a semiconductor device having aligned characteristics in realizing a mass production process and commercialization.
However, even when the sacrificial oxide film is formed in the previous step as the process protective film, or the sintering treatment is performed at 400 to 600° C. after Ti has been formed as the Schottky metal, stability of the forward characteristics, especially the barrier height φB of the diode cannot get a satisfactory result for a specification required for a practical use, and the variation in reverse leak current is problematically large.